The invention relates to contactless electronic labels which are affixed on products with a view to their identification and, more particularly, those which are intended to be affixed on products having a conductive surface.
Identifying products by “bar codes” is known, where said bar codes are printed directly on the products or on labels then stuck thereon.
These bar codes are read by an optical device when they are passed through at the cash desk and their decoding makes it possible to refer to a computer system with a view to knowing the price thereof, displaying it on a screen and adding it to the sum of the other products presented to the optical device.
One of the major drawbacks of bar codes is that they are fixed at the time of their printing and therefore do not allow tracking of the product over time, from manufacture to sale and, a fortiori, throughout the guarantee period.
Also, a proposal has been made to replace the bar codes by so-called electronic labels which comprise an electronic circuit having principally an electronic type memory for recording not only the information of a bar code but also other information relating to the “life” of the product, it being possible for certain of this additional information to be modified or updated during this life.
In simpler applications, the electronic labels are used as an anti-theft device by activating an alarm when the product goes past gates without having previously been presented at the cash desk for payment, the passage through at the cash desk with payment having the effect of deactivating the alarm.
Whatever the envisaged application, an electronic label 10 for contactless reading comprises (FIG. 1) an antenna 12 associated with an electronic circuit 14 in the form of an integrated circuit referred to as an electronic chip. The antenna 12 is implemented by a tuned circuit which consists of a coil 16 and a capacitor 18, the capacitor forming part of the electronic circuit 14.
The coil 16 is formed by a few turns 20 (FIG. 2) disposed on an insulating sheet 22, the ends 24 and 26 of the winding of the turns being connected to the terminals of the capacitor 18 disposed inside the electronic circuit. The electronic chip 14 is stuck onto the sheet 22.
Such an electronic label is read during its passage in front of a reading device which emits electromagnetic radiation detected by the antenna 12.
The signal emitted by the reading device comprises a high frequency signal which is modulated, for example amplitude modulated, by a low frequency signal which conveys the information to be transmitted, for example an instruction for reading the electronic memory.
The high frequency signal, for example at a frequency of 13.56 MHz, detected by the antenna 12 is used, after rectification and filtering, to supply the electronic circuit 14 with electrical energy whilst the low frequency signal is used, after demodulation and decoding, to recognise the instruction.
The instruction can consist of reading the electronic memory with a view to transmitting all or part of its content to the reading device by means of the antenna 12 which continues to receive a high frequency signal which is unmodulated but the load of which varies depending on a modulated signal representing the content of the memory read.
There results therefrom a modulation of the magnetic coupling between the antenna 12 and the antenna of the reading device which is detected and decoded by the latter.
The electronic label 10 is secured, for example by gluing, to the product to be identified or its packaging by means of the insulating sheet 22.
This type of electronic label, referred to as passive since it is powered by the energy of the high frequency signal, operates perfectly when the support of the electronic label is not electrically conductive but its operation is impaired, perhaps even impossible, when the support is made from an electrically conductive material since the electromagnetic radiation is absorbed by the conductive material of the support in the form of Foucault currents, so that the antenna of the electronic label no longer detects any signal and is obviously not supplied with energy by the high frequency signal.
In the case of a conductive support, a proposal has been made to use so-called active electronic labels which are supplied with electronic energy by a battery and which then operate as radiofrequency emitters. Such electronic labels, which are for example used on motor vehicle assembly lines, are bulky and expensive and can therefore not be fixed on small and/or inexpensive products.